A conventional art will be explained based on FIG. 8. FIG. 8 shows a cross-sectional configuration of an optical disk device in a conventional example. A laser beam emitted from a radiation source 1 such as a semiconductor laser is converted to parallel light 3 along the z-axis by a collimator lens 2. This parallel light 3 is incident on a reflecting surface 10A of a mirror 10 guiding light upward as reflected light 6. The reflected light 6 is converted to convergent light 8 by an objective lens 7. The convergent light 8 is transmitted through a surface 9B of an optical disk substrate 9, thus being focused on a signal surface 9A. This convergent light 8 enables signals to be recorded on the signal surface 9A or signals on a signal surface to be reproduced.
In this optical disk device, its thickness t (the distance from the lower surface 9B of the optical disk substrate to the bottom face of the mirror 10) is determined by the sum of t1+t2. The value of t1 is determined by adding the vertically movable width of the objective lens 7 to its thickness. The value of t2 is obtained by adding a margin (that is a thickness of a region that does not contribute to the reflection of the laser beam 3 at a lower part of the mirror 10 because the accuracy of the machined surface cannot be secured) to a beam diameter d before being guided upward. Considering a margin for an error in positioning the objective lens 7 (in the case where the z-axis is in the radial direction of an optical disk, also considering the maximum movable width of the objective lens 7 during tracking), the beam diameter d before being guided upward is larger than the aperture diameter of the objective lens 7. The aperture diameter is given by 2.times.NA.times.f, wherein NA and f represent a numerical aperture of the objective lens and a focal length, respectively. In DVD, the applied NA is 0.6. Since the resolution of the optical disk device is proportional to the square NA, the applied NA can not be further decreased. In order to prevent the objective lens 7 from hitting the substrate surface 9B, at least 1.3 mm of a working distance (the shortest distance between the substrate surface 9B and the objective lens surface) is required. Therefore, f is generally at least 2.4 mm in DVD. Consequently, an aperture of at least 2.times.NA.times.f=2.88 mm is required in DVD. The beam diameter d is about 3.3 mm including an adjustment margin of 0.4 mm, and the margin of the mirror 10 of 0.3 mm is added thereto, resulting in t2=3.6 mm. Furthermore, in the case of f=2.4 mm, t1 is 4.0 mm in view of its design. Thus, the thickness t of the optical disk device obtained is 7.6 mm.
In such a conventional optical disk device, there has been a problem that it is physically impossible to make the thickness t of the optical disk device less than 7.6 mm unless the working distance and the numerical aperture are changed.